1. Field of the Invention
The invention relates to a reflecting mirror for use in various optical systems and more particularly to a reflecting mirror having a plastic substrate and a reflection film made of silver or aluminum.
2. Description of the Related Art
A reflecting mirror including a glass substrate and a silver (Ag) reflection film or an aluminum (Al) reflection film evaporated thereon has been heretofore known as a reflecting mirror for use in various optical systems. The silver reflecting mirror including the silver reflection film, in particular, is superior in spectral reflectivity because of having higher reflectance in the visible light range and is also superior in polarization characteristics, as compared to other reflecting mirrors.
Since the silver reflecting mirror, however, is weak in durability, the silver reflecting mirror is configured to include a protective layer made of aluminum oxide or the like, which is formed on the silver film. However, the silver reflecting mirror including the protective layer has a problem: more specifically, the protective layer causes deterioration in the spectral reflectivity of the silver reflecting mirror.
Reflecting mirrors adapted to solve the above-mentioned problem include a reflecting mirror disclosed in Unexamined Japanese Patent Application Publication No. Hei 05-127004, for example. The reflecting mirror disclosed in the publication includes a glass substrate, and a stacked structure including a silver layer, an aluminum oxide layer and a high-refractive-index dielectric layer, which are stacked on the glass substrate in this sequence. The reflecting mirror having this configuration improves in spectral reflectivity throughout the visible light range.
Moreover, a plastic substrate rather than the glass substrate has recently come into wide use for the silver or aluminum reflecting mirror described above, in that the plastic substrate allows substantial reductions in cost and weight and permits manufacturing even the substrate of large size or complicated shape with relative ease (see Unexamined Japanese Patent Application Publication No. Hei 08-286007, for example).
However, the plastic substrate has moisture permeability, which may cause reaction between silver or aluminum which functions as the reflection film and moisture, thus cause corrosion of silver or aluminum, and result in a decrease in the reflectance of the reflection film. Furthermore, the plastic substrate is unreliable in terms of mechanical strength or wear resistance, because adhesion between the plastic substrate and the film formed thereon is weaker than adhesion between the glass substrate and the film formed thereon.
The reflecting mirror disclosed in Unexamined Japanese Patent Application Publication No. Hei 08-286007 includes dense films made of aluminum oxide or the like, which are formed on both surfaces of the reflection film, so as to prevent deterioration in the reflection film and thus improve environmental resistance. However, the reflecting mirror including the dense film has another drawback to be remedied, such as a decrease in the reflectance, although the decrease is slight. Moreover, it is desired that the reflecting mirror further improve in environmental resistance and adhesion.